二级索引的Cassandra查询非常慢

Question

We have a table with about 40k rows, querying on secondary index is slow(30 seconds on production). Our cassandra is 1.2.8. The table schema is as following:

CREATE TABLE usertask (
  tid uuid PRIMARY KEY,
  content text,
  ts int
) WITH
  bloom_filter_fp_chance=0.010000 AND
  caching='KEYS_ONLY' AND
  comment='' AND
  dclocal_read_repair_chance=0.000000 AND
  gc_grace_seconds=864000 AND
  read_repair_chance=0.100000 AND
  replicate_on_write='true' AND
  populate_io_cache_on_flush='false' AND
  compaction={'class': 'SizeTieredCompactionStrategy'} AND
  compression={'sstable_compression': 'SnappyCompressor'};

CREATE INDEX usertask_ts_idx ON usertask (ts);

When I turn on tracing, I notice there is a lot of lines like the following:

Executing single-partition query on usertask.usertask_ts_idx

With only 40k rows, it looks like there are some thousands of query on usertask_ts_idx. What could be the problem? Thanks

More investigation

I try the same query on our test server, it is much faster(30 seconds on prod, 1-2 seconds on test server). After comparing the tracing log, the difference is the time spending at seeking to partition indexed section in data file. On our production it takes 1000-3000 micro sec for each seek, on dev server it takes 100 micro seconds. I guess our production server has not enough memory to cache the data file so it is slow at seeking in data file.

Answer 1

I am presuming ts is a timestamp, in which case this is not a good candidate for a secondary index. The reason is that it's a high cardinality value (ie all values are essentially unique). This means you'll end up with almost one row in the index for each row in usertask --effectively resulting in a join operation. Joins are terribly slow on a distributed database. Since you haven't shown your query I'm not sure exactly what you're doing, but you'll need to rethink your model if you want to query based on time.